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![Galle's Catalogue of Comets, From 1860 to 1884 Translated From the "e;Astronomische Nachrichten,"e; nos. 2665 and 2666, by William C. Winlock and Reprinted From the Sidereal Messenger, for November, 1885, and January and February, 1886](https://bilder.buecher.de/produkte/69/69287/69287447m.jpg "Galle's Catalogue of Comets, From 1860 to 1884 Translated From the "e;Astronomische Nachrichten,"e; nos. 2665 and 2666, by William C. Winlock and Reprinted From the Sidereal Messenger, for November, 1885, and January and February, 1886")
Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Astronomers use sidereal time as a way to keep track of the direction in which their telescopes need to be pointed to view any given star in the night sky. Just as the Sun and Moon appear to rise in the east and set in the west, so do the stars. A sidereal day is approximately 23 hours, 56 minutes, 4.091 seconds, corresponding to the time it takes for the Earth to complete one rotation relative to the vernal equinox. The vernal equinox itself precesses very slowly in a westwards direction relative to the fixed stars, completing one revolution every 26,000 years approximately. As a consequence, the sidereal day is some 0.008 seconds shorter than the earth''s period of rotation relative to the fixed stars (sidereal period of rotation).